I was updated OTRS from 5 to 6.
However, garbled characters occurred with some type of mail after upgrading.
This mail of characters code type was ISO-2022-JP.
I have OTRS5 system. So I sent this email to the email address used by OTRS5.
This result was not garbled.
I watched databases characters to compare OTRS5 and OTRS6.
Both were stored in databases with ISO-2022-JP characters.
(I saw tables, OTRS5 -> article_plain, OTRS6 -> article_data_mime_plain)
This phenomenon doesn't happen all mails, it occurs with only a few mails.
I think that it has problems with the web display function of OTRS6.